In conventional pixel architectures of digital imagers shown in FIGS. 1 and 2, a photodiode 12 produces a photo-current proportional to light generated from x-ray exposure. The current is integrated on a parasitic capacitor 14. The accumulated charge is passed to a charge amplifier 16 via a switching transistor 18 through a data line 20. The amplifier 16 converts the charge to a corresponding voltage with a conversion coefficient depending on the capacitance of capacitor 22. Many rows of photodiodes can be read by the same set of amplifiers by consecutive selection of switching transistors. However, using a common data line for multiple pixels increases the parasitic capacitance at the input of the charge amplifier and gains inherent amplifier noise. The resulting deterioration of signal-to-noise ratio (SNR) limits the usability of the imager for low signal levels detected by the photodiodes.
To improve SNR at low signal levels, it will be desirable to have a signal gain in every one of the pixels without substantially affecting the amplifier noise. The commonly used pixel architectures shown in FIGS. 1 and 2 do not have pixel gain. FIG. 3 shows a pixel architecture of a prior art active pixel sensor with signal gain in the pixel. In this configuration, the transconductance of transistor 32 is used to gain the signal integrated on the capacitor 34. Reading out the pixel signal through transistor 36 does not recover the bias voltage across the photodiode 38 needed for normal operation. Therefore, a separate reset operation is needed to restore that voltage. It is done by enabling transistor 40 so the voltage across the photodiode 40 is defined by the difference between VREF1, and VREF2. A separate reset of the photodiode voltage adds time to the imaging cycle and complexity to the imager. Furthermore, the signal transfer function shown in FIG. 3 is non-linear due to the non-linearity of the voltage-to-current dependency of transistor 32 and 36. Variations in the parameters of the transistors because of temperature or technology fluctuations also greatly affect the signal transfer function of the pixel and lead to image artifacts.
In conventional pixel architectures shown in FIGS. 1-3, the voltage across the photodiode changes during signal integration. Allowing the voltage across the photodiode to change during signal integration causes sensitivity modulation and non-linear behavior. When the signal in some parts of the imager is too strong, it leads to saturation of the photodiode and collapsing of the voltage across it. This effect complicates the use of continuous photodiodes, which are being increasingly employed in imaging devices.